There have been many cases at a construction or building site where the earth must be deeply excavated to form a hole having a depth which is too long relative to its diameter. For example, there have been cases for excavating the earth to form a hole in which an anchor supporting a steel tower is embedded, a hole in which a water purifier tank is embedded, a hole for ground-making and a hole for well sinking. In such cases, the hole should generally have a depth which is too long, e.g. ranging from 15 m to 20 m, relative to its diameter, e.g. about 5 m.
In deep excavating work, there is conventionally employed a deep excavator having a telescopic mechanism comprising a stretchable arm fixed to a boom wherein a clamshell bucket (hereinafter referred to as a bucket) is coupled to the tip end of a top arm of the stretchable arm. In this arrangement of the deep excavator, most of the excavating mechanism comprises the stretchable arm which is fixed to the tip end of the boom has at least two stages of arms in which the bucket suspended from the top arm is hung to reach the bottom of the hole. However, in such an arrangement, arms of the stretchable arm must be extended and contracted synchronously with one another. This leads to a complex mechanism for extending and contracting each arm.
In the conventional mechanism for extending and contracting each arm, a wire or chain is entrained around or extended between each arm whereby each arm is extended and contracted synchronously with one another by such wire or chain. In such a mechanism, it is possible to smoothly extend or contract each arm of the stretchable arm but the wire or chain must be entrained around or extended between each arm, which makes the arrangement of the wire or chain complex. Furthermore, since the wire or chain for contraction of each arm as well as extension of each arm must be entrained around or extend between each arm, at least two wires or chains are required for one arm, which leads to a complex arrangement of the wires or chains. In such an arrangement of wires or chains, the wires or chains are liable to be exposed outside the stretchable arm which is not preferable in view of the external appearance. There is a likelihood that earth and sand is stuck to the wires or the chains, which causes abrasion or is troublesome to the mechanism.
Accordingly, there is proposed a mechanism for extending and contracting a stretchable arm using hydraulic power generated by hydraulic cylinders which are incorporated into the stretchable arm. However, in the conventional excavator, it is necessary to provide high pressure application hoses (hereinafter referred to as pressure hoses) on each hydraulic cylinder coupled to each arm for supplying oil under pressure to each hydraulic cylinder. If the pressure hoses are loosened, they become intricate in the stretchable arm, which makes the mechanism complex. Furthermore, the pressure hoses may become fatigued and broken if they are used for a long time. Still furthermore, if each arm is operated by a plurality of hydraulic cylinders, the stretchable arm cannot be extended or contracted at high speed and the extending and contracting speeds are slower compared with using wires or chains.
In the conventional excavator, if the wire or chain is used for extending and contracting the stretchable arm, the extending and contracting speeds are fast but the mechanism thereof is complex and there is the drawback in that the wire or chain is exposed outside the stretchable arm. Whereas, if the hydraulic cylinder is used for extending and contracting the stretchable arm, there is an advantage in that the external appearance of the stretchable arm is simple and the wire or chain is not exposed outside the stretchable arm but there is the drawback that the extending and contracting speeds are slow.